1. Field of the Invention
The present invention relates to balls for a ball-point pen which are to be fitted in a pen tip of the ball-point pen.
2. Description of the Related Art
A pen tip of a gel-ink ball-point pen typically has a construction as shown in FIG. 7. In FIG. 7, a pen tip body 10 has a hollow inside portion and a tapered tip portion provided at a front end thereof, and a writing ball (hereinafter referred to simply as "ball") 11 is rotatably fitted in the tip portion. An ink reservoir tube (not shown) is connected to a rear end of the ball tip body 10, so that ink is supplied to the hollow inside portion from the ink reservoir tube and then to the ball 11 through an ink flow path 12.
More specifically, the tip portion of the pen tip has a ball socket 13 communicating with the ink flow path 12, and the ball 11 is rotatably retained in the ball socket 13 by a crimped rim 14 provided at an opening of the ball socket 13 as shown in FIG. 8. The ball socket 13 has a concavely tapered innermost surface 15, on which five ink channels 16 are radially provided as shown in FIG. 9 to ensure a sufficient ink flow.
When a line is to be written on a paper surface 22 with the ball-point pen having the above pen tip, the ball 11 provided at the pen tip is pressed against the paper surface 22 with the pen tip tilted with respect to the paper surface 22 as shown in FIG. 10. At this time, the ball 11 is displaced diagonally upward (in an arrow direction B as seen in FIG. 10) within the ball socket 13. The ink is introduced into the ball socket 13 from the hollow inside portion of the pen tip body 10 through the ink flow path 12 and the ink channels 16, and flows out through a gap between a wall surface of the ball socket 13 and the ball 11 (in an arrow direction C as seen in FIG. 10) by gravity. While the ball 11 is rolled across the paper surface 22, the ink flows through the surface of the ball 11 to be laid down on the paper surface 22 for the writing.
Typically used as a material for the ball 11 is a cemented carbide. For formation of the ball 11, the cemented carbide is shaped into a spherical body, which is then polished. More specifically, a cemented carbide obtained by sintering a mixture of particles 17 of a hard intermetallic compound such as WC and a binder metal 18 such as Co is shaped into a spherical body, and a surface 19 of the spherical body is mirror-finished by polishing.
In the case of the aforesaid conventional ball 11, however, gaps between the hard intermetallic compound particles 17 are filled with the binder metal 18, so that minute fragments of the hard intermetallic compound are exposed from binder metal portions 18 on the surface 19 of the ball 11 after the ball surface 19 is mirror-finished. During use of the ball-point pen, the binder metal portions 18 are abraded, and the minute fragments 20 fall off as shown in FIG. 12. The minute fragments 20 having thus fallen off act as an abrasive thereby to abrade the pen tip body 10.
As the soft binder metal portions 18 between the hard intermetallic compound particles 17 are abraded, edges 21 of the hard intermetallic compound particles 17 are exposed on the polished ball surface. When the ball 11 having the edges 21 of the hard intermetallic compound particles 17 exposed on its surface is rotated with a writing pressure being applied thereto, the edges 21 have a cutter-like action on the pen tip body 10, so that the tapered surface 15 of the ball socket 13 of the pen tip body 10 is abraded thereby to cause the ball 11 to sink into the pen tip body 10 as shown in FIG. 13.
The abrasion of the tapered surface 15 of the pen tip body 10 causes the ink channels 16 to clog, thereby reducing the ink flow. Further, a friction resistance between the ball 11 and the pen tip body 10 is increased with a greater contact surface therebetween, so that the ball 11 cannot smoothly be rotated. Moreover, the sinkage of the ball 11 into the pen tip body 10 causes the crimped rim 14 of the pen tip body 10 to abut against the paper surface unless the pen is oriented at a greater angle with respect to the paper surface. This results in skipping or thinning of written lines. That is, the angle of the pen with respect to the paper surface 22 which ensures smooth writing (writing permissible angle) becomes greater. With a greater writing permissible angle, the displacement of the ball 11 in the writing (movement of the ball 11 in the arrow direction B as seen in FIG. 10) is reduced, whereby the gap between the wall surface of the ball socket 13 and the ball 11 through which the ink flows out is narrowed to further reduce the ink flow. With the conventional ball 11, therefore, the pen tip body is abraded at an early stage of use of the ball-point pen, so that writing irregularities such as skipping or thinning of written lines and deterioration of writing feeling occur earlier.
In view of the foregoing, it is an object of the present invention to provide a ball for a ball-point pen which can alleviate the abrasion of the pen tip body to ensure a satisfactory writing feeling for an extended period.